A heat recovery steam generator (“HRSG”) is a heat exchanger that recovers heat from a hot gas stream. It produces steam that can be used in a process or used to drive a steam turbine. A common application for an HRSG is in a combined-cycle power station, where hot exhaust from a gas turbine is fed to an HRSG to generate steam which in turn drives a steam turbine. HRSGs often consist of three sections: an LP (low pressure) section, a reheat/IP (intermediate pressure) section, and an HP (high pressure) section. Each section has a steam drum and an evaporator section where water is converted to steam. This steam then passes through superheaters to raise the temperature and pressure past the saturation point. “Low pressure” can be defined, for example, as a pressure that is less than, equal to, or not greatly above, atmospheric pressure, while “high pressure” can be defined, for example, as a pressure that greatly exceeds atmospheric pressure. “Intermediate pressure” would then be a bewteen these two levels.
FIG. 1 is a schematic drawing of a prior art double flow, high pressure (“HP”), non-condensing (“DFNC”) turbine 10 with multiple stages (not shown). Turbine 10 includes a casing 11 with an inlet 22, two high pressure sections 13 and 15, and two exhaust outputs 12 and 14. Connected to turbine 10 is a two-level heat recovery steam generator (“HRSG”) 16 with a high pressure section 18 and an intermediate pressure section 20. As is typical with heat recovery steam generators, HRSG 16 recovers heat from a hot gas stream (not shown) and generates steam that is used to drive steam turbine 10. This steam is fed into turbine 10 through inlet 22, which is connected to HRSG 16 through pipe line 23.
Turbine 10 has a very high temperature at its inlet 22 and an exhaust temperature of about the same value at its exhaust outputs 12 and 14, when running at full speed, no load (“FSNL”). The exhaust outputs 12 and 14 are connected to a valve 24 through pipe line 25. The exhaust pressure is controlled by valve 24 and set at a constant value.
Typical turbine conditions will depend on the needs of the customer using the turbine. Thus, for example, where turbine 10 is used in a desalination plant application, it might have an inlet temperature of about 1015° F., an exhaust temperature of about 980° F., an exhaust pressure of about ˜40-50 psia (pounds-force per square inch absolute, i.e., gauge pressure plus local atmospheric pressure) and a pressure drop between inlet and exhaust at full load of approximately 1400 psia to 40 psia, resulting in a large expansion line. The expansion line is a thermodynamic measure of the turbine efficiency for a given pressure ratio. The biggest delta in energy between inlet and exhaust conditions is the highest efficiency. Each design is optimized for a given pressure ratio. When a different pressure ratio is applied, the efficiency is not optimum anymore. The worse case is FSLO. At this load, the pressure ratio and expansion line are reduced to its minimum and the efficiency is the lowest (i.e., the inlet and exhaust energies are about the same). This results in high temperatures from inlet to exhaust. The “process steam” exiting exhaust outputs 12 and 14 is typically used in a process of some sort operated by a customer connected to valve 24 by a further pipe line 27.
Also connected to pipe line 27 is a pipe line 21 that is connected to intermediate pressure section 20 of HRSG 16. Line 21 is used in a customer process, and thus, it is not used to produce power in the steam turbine 10. There are certain desalination plants that require accurate steam condition into the process, steam turbine exhaust conditions can vary a lot from its expected conditons due to manufacturing, installation, operation, etc. Line 21, in this particular case, is used to achieve certain conditions by mixing with steam exhaust flow at full load or normal operation. During FSNL, line 21 is not required for the desalination process (the desalination process is established at higher loads), and can be used as “cooling” into the steam turbine inlet. A lower inlet temperature drives a lower exhaust temperature, as compared against HP steam. Both stean productions (HP and IP) are available during FSNL.
When turbine 10 runs at full speed, no load, the flow of steam decreases greatly at the exhaust outputs 12 and 14 of HP sections 11 and 13, and pressure begins to feed back to the up-front stages within turbine 10. As a result of this feed back of pressure, the up-front stages of turbine end up being at about the same pressure as the exhaust outputs 12 and 14 (i.e., ˜40 psia), and thus, no flow of steam occurs throughout turbine 10. Only windage heating occurs due to the extremely short expansion line so that the stages of turbine 10 become exposed to high temperatures and possible hardware damage from such high temperatures.